For many years the primary material used to control water hardness in detergent products has been sodium tripolyphosphate at levels of approximately 50% by weight of the finished detergent product. Within the past few years the use of high levels of sodium tripolyphosphate has come under scrutiny because of the suspicion that soluble phosphate species accelerate the euthrophication or aging process of water bodies. This eutrophication is ordinarily evidenced by the rapid growth of algae in the water body. Sodium tripolyphosphate exists as a molecule containing 5 atoms of sodium, 3 of phosphorus, and 10 atoms of oxygen. When utilized as a detergent builder the sodium tripolyphosphate molecule sequesters as a soluble species one molecule of calcium or magnesium cations per molecule of tripolyphosphate anion. In other words, sodium tripolyphosphate sequesters calcium and magnesium ions on a 1:1 mole basis. The calcium or magnesium tripolyphosphate species is relatively stable in a wash solution, thus preventing the water hardness from reacting with anionic detergents which in turn gives better cleaning. The calcium or magnesium tripolyphosphate species exists essentially as a single trivalent negative anionic species in the wash solution. This calcium or magnesium trivalent anion does not precipitate in the course of the wash. Sodium pyrophosphate has generally been considered to be an equivalent builder when compared to sodium tripolyphosphate. It has also been suspected that sodium pyrophosphate is the equivalent of sodium tripolyphosphate in promoting the eutrophication as has been discussed above.
Sodium pyrophosphate contains one less atom of phosphorus than does sodium tripolyphosphate. The foregoing is reflected in the empirical formula of sodium pyrophosphate which is 4 atoms of sodium, 2 atoms of phosphorus and 7 atoms of oxygen. Sodium pyrophosphate is also known to form a 1:1 molar complex with calcium or magnesium ions. The species resulting from the sequestration of calcium or magnesium by the tetravalent negative pyrophosphate anion in the calcium or magnesium divalent negatively charged pyrophosphate complex. This calcium or magnesium pyrophosphate complex is sufficiently stable in the wash solution to prevent the water hardness cations from interfering with the detergency process.
Aside from a molecular weight advantage which allows more moles of the pyrophosphate salt to be present in a composition at a given weight fraction of phosphate than tripolyphosphate, there would appear to be little difference between using pyrophosphates and tripolyphosphate in detergent products. However, such is not the case, one curious difference exists between the pyrophosphate anion and the tripolyphosphate anion and that is the ability of the former to precipitate as the dicalcium pyrophosphate salt under normal wash conditions. The precipitation of pyrophosphates has been sought to be avoided in wash solutions as the salt formed has a tendency to build up on fabrics and exposed machine surfaces.
Furthermore the precipitation of calcium pyrophosphate is unpredictable under normal wash conditions where such factors as the total hardness, the pH, and the ratio of calcium ions to magnesium ions may vary from load to load.
Johnson states in U.S. Pat. No. 2,381,960 issued Aug. 14, 1945 that water-hardness may be reduced by adding pyrophosphates to the solution containing the hardness after a supplemental alkaline material such as sodium orthophosphate, alkali metal hydroxides and carbonates, soap and sodium silicate having an SiO.sub.2 :Na.sub.2 O ratio greater than 1.5 have been added to the solution. Kepfer in U.S. Pat. No. 2,326,950 issued Aug. 17, 1943, discloses that pyrophosphates can be used to control water hardness if the pyrophosphate is added to the solution containing the hardness prior to the addition of supplemental alkaline materials such as sodium borate, sodium metasilicate and disodium dihydrogen phosphate. In both Johnson, supra, and Kepfer the second component (Johnson-pyrophosphate and Kepfer-supplemental alkaline material) is to be added to the solution prior to the formation of a macroscopic precipitate.
It can thus be seen that pyrophosphate has the potential of being a much more effective detergency builder than have the tripolyphosphate salts. The pyrophosphate tetravalent anion has a high association constant with the first calcium ion with which it associates. This first association product is the monocalcium pyrophosphate divalent anion. The divalent anion has a very small association constant with the second calcium ion to form the dicalcium pyrophosphate salt which is electrically neutral. In the absence of any material which makes the dicalcium pyrophosphate more stable one of the associated calcium ions will be free of disassociate and to seek a more stable association such as with body soil on the fabrics or with the anionic detergent. The main purpose of controlling calcium ions whether free or associated in a weak complex is to prevent the last mentioned reaction with the detergent or soiled fabric from occurring. As the pyrophosphate anion strongly holds one mole of calcium ion per pyrophosphate anion it has been common practice to attempt the first association (sequestration) on a mole for mole basis. If, however, the pyrophosphate anion can be induced to strongly associate (precipitate) with two moles of calcium ion the pyrophosphate level used could be reduced substantially. Preferably some free pyrophosphate tetravalent anion will be present in the wash for its value in peptizing clay soils.
It has been discovered as disclosed in commonly assigned copending U.S. application Ser. No. 618,303 by R. W. Benson, S. D. Cherney and E. J. Collier filed on Sept. 29, 1975 and incorporated herein by reference, that alkali metal pyrophosphates can be formulated into a detergent composition in a manner such that the pyrophosphate builds by first associating with and then precipitating up to two moles of calcium per mole of pyrophosphate. However, it has long been the experience of those skilled in the art that pyrophosphates have not been very effective for whiteness maintenance. Good `whiteness maintenance` results from the ability of a detergent composition to prevent dirt dispersed in a wash liquor, for instance having been washed out of soiled articles, from being re-deposited upon clean or cleaned articles.